Sang-Im Yoo

Melt-processed light rare earth element-Ba-Cu-O

Unlike Y123 which forms only a stoichiometric compound, the light rare earth elements (LREs: La, Nd, Sm, Eu, Gd) form a solid solution . The presence of such solid solution caused a depression in the superconducting transition temperatures , particularly for La123, Nd123 and Sm123 when they are melt processed in air. Recently, we have found that the of these LRE123 superconductors can greatly be enhanced when they are melt processed in a reduced oxygen atmosphere. Furthermore, values of these superconductors were larger than that of a good quality Y123 superconductor in high magnetic fields at 77 K. In this article, on the basis of our study over the last several years, the melt processes for LRE - Ba - Cu - O are described, the microstructural and superconducting properties of the superconductors are reviewed and the flux pinning mechanism is also discussed.

Melt processing of high-Tc Nd-Ba-Cu-O superconductors in air

Abstract We have succeeded in fabricating Nd-Ba-Cu-O bulk superconductors, which exhibit high T c and a sharp superconducting transition, by the melt process in air without additional high temperature annealing in a reduced oxygen atmosphere. The key to this achievement is the use of Ba-rich Nd 4−2 z Ba 2+2 z Cu 2− z O 10− δ as a precursor. Fabricated bulk samples exhibit onset T c values of 94–95 K and the secondary peak effect in the M-H loop like the case of Nd-Ba-Cu-O superconductors produced by melt processing in a reduced oxygen atmosphere. The peak position and the irreversibility field for the best sample were about 1.2 T and 5 T at 77.3 K, respectively, and the estimated J c at the peak field was about 2.3 × 10 4 A cm −2 .

Microstructures and superconducting properties of melt-processed (RE,RE′)BaCuO

Abstract We have studied the microstructures and superconducting properties of melt-processed (RE,RE′)ue5f8Baue5f8Cuue5f8O composite bulks (RE and RE′ are two different rare earth elements selected from a group of Nd, Sm, Eu, Gd and Y). It was found that the peritectic decomposition temperatures ( T m ) of the (RE,RE′)ue5f8Baue5f8Cuue5f8O composite increased linearly with increasing the average ionic radius between two different rare earth elements. Large grain growth was observed in almost all the (RE,RE′)ue5f8Baue5f8Cuue5f8O bulks except those containing Y, when they were melt-processed in a reduced oxygen atmosphere. Compositional analyses revealed that the ratio of RE to RE′ in the (RE,RE′)Ba 2 Cu 3 O 7− d matrix phase was almost the same as that of the nominal composition, showing that the mixture of the two different rare earth elements was very uniform. All the samples showed onset superconducting transition temperatures ( T c ) exceeding 93 K with a sharp transition, and exhibited a secondary peak effect in the M - H loops.

Phase relation in Y211–Y123–Ag system and morphology of silver in Y123 crystal

Abstract Phase relation in the Y 2 BaCuO 5 (Y211)– YBa 2 Cu 3 O 7− x (Y123)–Ag system, which is important for the fabrication of Y123/silver composite, and the morphology of silver particles dispersed in the Y123 matrix were investigated. The formation temperature of Y123 monotonously decreased to about 970°C as increasing Ag content up to a pseudo-monotectic composition and kept constant at the temperature with further Ag addition. The Y123/silver composite could be obtained at the temperatures lower than 970°C. These results suggest that Y123 phase is solidified by monotecto-peritectic reaction; Y211+L1 (Ba–Cu rich liquid)=Y123+L2 (Ag rich liquid). The morphology of silver particles in Y123 was altered by the addition of excess Y211. In the samples without excess Y211, silver particles dispersed in Y123 were disk-like shaped. On the other hand, those became spherical in the samples with excess Y211. The capturing mechanism of L2 liquid is considered to be the Uhlmann, Chalmers, and Jackson (UCJ) type pushing one. The amount of residual Y211 particles at the growth interface is considered to cause the morphological difference of captured silver between samples with and without excess Y211.

Melt-processed LRE-Ba-Cu-O superconductors and prospects for their applications

Abstract We have recently found that control of the oxygen partial pressure (PO2) during melt processing, named the oxygen-controlled melt-growth (OCMG) process, is critical for obtaining a high superconducting transition temperature (Tc) in the light rare earth (LRE)–Ba–Cu–O (LREBCO) superconductors particularly for Nd, Sm and Eu. Further, compared to a good melt-processed Y–Ba–Cu–O (YBCO) bulk superconductor, LREBCO bulks exhibit larger critical current density (Jc) in high magnetic field and a much improved irreversibility field (Hirr) at 77 K, implying that more effective flux pinning can be realized in a commercially feasible way. In this paper, properties and characteristic flux pinning of OCMG-processed LREBCO (LRE: Nd,Sm,Eu) superconductors are described on the basis of our study during the last several years. We also present the prospects for bulk-type applications, such as the magnetic bearings, flywheels and magnetically levitated (MAGLEV) trains.

Effects of oxygen content on the superconducting properties of melt-textured LRE123 superconductors

Abstract The effects of oxygen content on the superconducting transition temperatures, T c , and magnetic properties of melt-textured LRE123 (LRE ue5fb Nd, Sm) superconductors were studied. The oxygen content could be varied by changing the annealing temperature, T a , in that the oxygen deficiency, δ, monotonically decreased with decreasing the T a from 600°C to 300°C. A further decrease of T a to 275°C showed no appreciable change. In both systems the highest T c was obtained for the samples annealed at 300°C. Magnetic properties were also strongly dependent on the oxygen content, showing that the control of oxygen content is critical for achieving good pinning properties.

Fabrication of large single-domain Sm123 superconductors by OCMG method

Abstract Melt processing of Sm-Ba-Cu-O bulk superconductors was studied to find the optimum conditions for fabricating a large single-domain superconductor. First, the holding time at the maximum temperature for the partial melting is important for the fabrication of a uniform bulk, especially for a large diameter. When the holding time at the maximum temperature during partial melting was not enough, two separated regions were observed, which can be diminished by sufficient holding time. Next we studied the optimum conditions for grain growth, varying the temperature gradient and cooling rate. The processing condition with which we have succeeded in growing a single-domain SmBa 2 Cu 3 O y (Sm123 hereafter) with dimensions of 35 φ × 15 mm 2 is a temperature gradient of 7.5 °C cm −1 with a slow cooling rate of 0.33 °C h −1 .

Anisotropic magnetization properties of an single crystal

The magnetization properties of a flux-grown (Nd123) single crystal were investigated by measuring DC magnetization hysteresis (M-H) loops in a wide temperature range (10-92 K) and for applied fields parallel and perpendicular to the c-axis of the sample. For both field directions, temperature-dependent fishtail peaks in the M-H loops are clearly observed at high temperatures above 70 K, and all the data could be scaled to a single master curve with the hysteresis loop width versus H plot when they are normalized with the peak field and its magnetization value , respectively. This indicates that the origins of the peak effect and the pinning are identical for this temperature range. We have also found that and the irreversibility field scale with the reduced temperature, (: critical temperature), and follow the relationship with a power n of 1.1-1.3. The anisotropy parameters of and of were deduced from the angular dependence of and at 88 K. We emphasize that all the present results support our proposal that the peak effect in the Nd123 originates from the localized Nd-Ba substituted regions with depressed .

A.c. susceptibility measurements in the presence of d.c. magnetic fields for Nd-Ba-Cu-O superconductors

We report the temperature and magnetic field dependence of the complex a.c. susceptibility with bias d.c. magnetic fields for melt-processed Nd - Ba - Cu - O superconductor. The onset temperature of the real part of a.c. susceptibility shifted to a lower temperature with increasing d.c. magnetic field. The superconducting transition temperature determined by d.c. magnetization measurements did not shift appreciably to a lower-temperature region with increasing d.c. magnetic field. The distinction between and indicates that the a.c. susceptibility measurements detect the energy dissipation generated by the motion of flux lines. We have also measured flux profiles and found that there was no appreciable change in flux penetration below and above the peak field, which suggests that the peak effect in Nd - Ba - Cu - O is not due to the phase transition in the flux line lattice.

Refinement of Nd-422 phase trapped in a Nd1+δBa2-δCu3O7-γ superconducting matrix

The possibility of controlling Nd-422 phase dimension and morphology, both in the partially melted state and in the final superconducting matrix, was investigated. Nd{endash}Ba{endash}Cu{endash}O samples were prepared by the melt powder melt growth (MPMG) technique. Precursor materials containing Nd as either Nd{sub 2}O{sub 3} or Nd-422, formed during precursor quenching, were used. The role of Pt in Nd-422 refining was analyzed. The effect on Nd-422 nucleation and growth, of stoichiometries with increasing Nd-422 molar excesses (0{percent}, 10{percent}, 20{percent}, and 40{percent}) combined with different heating processes, up to the partially melted state, were examined. Uniformly distributed needle-like particles (1 {mu}m diameter and 10{endash}15 {mu}m length) were obtained at 1120{degree}C and resulted in a further refined distribution trapped in completely processed samples.